Space borides

AH of the alloys Hsted in Tables 4 and 5 are austenitic, ie, fee. Apart from and soHd-solution strengthening, many alloys benefit from the presence of carbides, carbonitrides, and borides. Generally the cubic MC-type monocarbides, which tend to form in the melt, are large and widely spaced, and do not contribute to strengthening. However, the formation, distribution, and soHd-state reactions of carbides are very important because of their role... 

The cubic UB, 2-type boride structure with space group Fm3m can be described on the basis of a B,2-cubooctahedron (see Fig. 1) . The association of the B,2-poly-hedra by oriented B—B bonds gives rise to a three-dimensional skeleton with boron cages. Formally, the arrangement of the B,2-units and of the metals atoms is of the NaCl-type. Each metal is located in the center of a B24-cubooctahedron. 

In a crystal-pulling procedure using a tri-arc furnace (Fig. 2), a resistor box, a d.c. power supply (300 A, 80/40 V) and a set of water-cooled power cables are used to bring power and water to the electrodes. The upper part of the furnace is equipped with three equally spaced copper cathodes, to which are fixed W-Rh electrodes. The upper part (cathode) is separated from the lower part (anode) by a transparent quartz glass tube. In the bottom of the furnace there is a tapered opening for a water-cooled copper hearth containing the boride melt. All parts of the furnace are also water... 

The hardest of the transition-metal borides are the diborides. Their characteristic crystal structure (Figure 10.6) consists of plane layers of close-packed metal atoms separated by plane openly-patterned layers of boron atoms ( chicken-wire pattern). If the metal atoms in the hexagonal close-packed layer have a spacing, d, then the boron atoms have a spacing of d/V3. 

Calcium hexaboride. An introductory example is represented by the calcium boride (CaB6) in which, however, boron octahedra are not condensed with each other but connected via normal (2e, 2c) bonds. A scheme of the structure is shown in Fig. 4.29. It is cubic, space group, Pm3m, N.221 with ... 

The process of forming boron-boron bonds is carried on further in aluminum boride, AIB, which has a very simple hexagonal structure, consisting of hexagonal layers of boron a,toms, like the layers of carbon atoms in graphite, with aluminum atoms in the spaces between the layers (Pig. 11-15). The B—B bond length is 1.73 A, corresponding to n = 0.66 that is, two valence electrons per boron atom are used in the B—B bonds, which are two-thirds bonds. 

It has been reported that ScBi2 has a tetragonal modification of the UBi2-type structure with lattice parameters of a = 5.22 A and c = 7.35 A with the space group of I4/mmm (Hamada et al., 1993 Paderno and Shitsevalova, 1995). Due to the small size of scandium as compared to the other rare earth atoms, scandium phases have been observed to form anomalous higher boride structures compared to the heavy lanthanides and yttrium, as will be discussed later in Sections 9 and 11. Small amounts of metal replacement for Sc in Sci xMxBi2 (x as small as 0.1, M = Y, Tm, Lu) have been reported to stabilize the structure in the normal cubic UBi2-type. 

Most of the higher borides described in this work were discovered within the space of 10 years, yet what other new attractive cluster compounds are waiting to be discovered along with their exciting and unexpected magnetic, electrical, and thermal properties ... 

Lack of space prevents descriptions of a number of additional types of boron compounds such as the numerous interstitial metallic borides, the subhalides (of type B2CI4), the organoboron compounds (such as B(CH3)3), and the boron-nitrogen addition compounds, many of which have been prepared by H. C. Brown to study the effects of structures on the stability of Lewis acid-base adducts. ... 

Owing to the great number of structure types represented among the binary and ternary borides, it is not possible to present in this article even a simple description of all known structure types. The following presentation deals mainly with binary boride structmes. Ternary boride structures show many similarities with those of binary boride phases but cannot be treated in any detail in the present paper because of space limitations. Binary and ternary boride structures are treated in more detail elsewhere. It is convenient to describe here... 

G.16 W. B. Pearson. A Handbook of Lattice Spacings and Structures of Metals and Alloys (New York Pergamon Press, 1958). A most useful source of information. Gives the crystal structures of intermediate phases, and the variation of lattice parameter with composition in solid solutions, of binary and ternary alloys. Also gives the crystal structures of metal borides, carbides, hydrides, nitrides, and binary oxides. 

Boron Nitride, Metal Borides, and Related Spedes.— Alo.o6BeB3 05, Le. BeB3 belongs to the space group P6/mmm, and contains B12 icosahedra and other polyhedral units of Be and B atoms. The linkages between the polyhedra resemble those in /3-rhombohedral boron. Aluminium atoms occupy interstitial... 

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